The present invention relates to a digital radio relay system, in particular, to a circuit unit for manipulating the auxiliary signals such as the so-called order wire signal and so forth, which are used for the the purpose of supervisory and/or maintenance operations of the system.
In a long haul digital radio transmission network, signals are relayed by a number of repeating stations installed between the terminal stations, wherein the auxiliary signal such as the so-called order wire signal, for example, is transmitted together with a digital main signal. Included in the auxiliary signals are the order wire signal for providing the stations with local telephone communications capability and others for monitoring and informing of the failure of the transmission circuit or a power supply of the relay equipment.
The auxiliary signal used in the digital radio transmission systems in the prior art has been an analog signal created by a moderate frequency modulation (FM) of the carrier radio frequency. The carrier wave is typically modulated by a main signal to be transmitted in the manner of a time-division multiplexed (TDM) digital signal by using PSK (phase shift keying), QAM (quadrature amplitude modulation), etc. prior to FM using the auxiliary signal.
However, the trend in modern systems for digital radio transmission is directed toward fully digitalized transmission, that is, both the main and auxiliary signals should be digital. This is because the concurrent operation of different modulation techniques such as FM and PSK or QAM, for example, in a digital radio transmission system makes the design and maintenance of the system complicated, and further, it is difficult to apply FM to the carrier which has been subject to PSK or QAM, particularly when the number of phases in PSK or the levels of QAM becomes large to increase the number of transmission channels on the carrier.
FIG. 1 is a conceptual block diagram for illustrating the operation of a digital radio transmission network comprising terminal stations 1 and 3 and the repeating stations 2, 2', etc. and FIG. 2 is block diagram showing an exemplary configuration of a relay equipment used for the repeating stations 2, 2', etc. in a prior-art fully-digitalized radio transmission network as shown in FIG. 1.
Each of the repeating stations, i.e., the relay equipment shown in FIG. 1, comprises a radio unit and an auxiliary unit 4, wherein the radio unit includes a receiver RX and a transmitter TX, while a hybrid circuit H is shown to represent the auxiliary unit 4 in FIG. 2. Such a network is usually bidirectional, however, FIG. 1 represents a situation where a signal is transmitted from the terminal station 1 to the terminal station 3. Therefore, in FIG. 1, in the terminal station 1 only a transmitter TX is illustrated, while only a receiver RX is illustrated in the terminal station 3.
In the following description, the order wire signal is taken to stand for the auxiliary signals, except when referring to other auxiliary signals.
Referring to FIGS. 1 and 2, a TDM transmission signal is sent from the terminal station 1 via the radio transmission line 100 to the repeating station 2, where it is separated into the main signal and the order wire signal by the receiver RX, and the main signal is directly sent to the transmitter TX. The order wire signal OW is input to the decoder 5 and converted to an analog audio frequency signal, and then, sent to telephone equipment (not shown). Another OW signal input from the telephone equipment in the repeating station 2 is sent to the encoder 6 via hybrid circuit H and converted to a digital signal, and then transmitted from the transmitter TX to the stations on the downstream transmission line together with the main signal in TDM mode. The output of the decoder 5 is branched to the hybrid circuit H, hence, the OW signal sent from a precedent station must be transmitted to all subsequent stations equally.
However, if a failure occurred in the auxiliary unit 4 shown in FIG. 2, the OW signal sent from a precedent station is received at the relevant station but it can not be transmitted to subsequent stations any longer. Accordingly, possible intercommunications by OW signals are limited between sections divided by the failed repeating station. It should be noted that the main signal separated by the receiver RX is, of course, sent to the transmitter TX, and transmitted to the subsequent stations, even when such failure occurs in the auxiliary unit.
In the prior art digital radio relay equipment, the auxiliary unit 4 is duplexed as is the radio unit in order to prevent the OW from interruption by failure of the auxiliary unit, however, this duplex configuration results in the reduction in the cost-performance characteristics and space factor of the equipment.